The present invention relates to urea-based products that are released over a controlled period of time and to methods of making the same. The invention is particularly concerned with controlled release urea-based feed supplements for animals and controlled release urea-based plant nutrients.
Urea in particulate form is an excellent and economical source of nitrogen to stimulate plant growth. Unfortunately, it is rapidly soluble in water. In spring rains, for example, the water soluble plant nutrient is quickly flushed away thereby (a) depleting its efficacy as a plant food, (b) potentially causing phytotoxicity to the plants, and (c) causing the nutrients to migrate to and contaminate the ground water and local wells and streams.
Researchers have for a long time attempted to develop particles which release nutrients at a rate about equal to their uptake by plants to minimize phytotoxicity and maximize use efficiency. Improved release control has been achieved primarily with nitrogen in the form of urea by substantially reacting it with aldehydes to form insoluble products such as ureaform, which must chemically decompose in the soil before nitrogen becomes available for utilization by plants. Another method consists of physically coating fertilizer granules with solidified water insoluble melts. Various materials have been used as coatings, including sulphur, paraffin waxes, vegetable oils and plastics. These coatings must be broken down by erosion, internal vapor pressure, microbes, or attrition before the contained nutrients become available.
Urea in particulate form is also an excellent and economical source of non-protein nitrogen (NPN) for use as a supplement in the feeding of ruminant animals, e.g., cows, cattle, sheep, goats, etc. Microorganisms found in the rumen or first stomach of ruminants allow the animal to manufacture protein amino acid from simple nitrogen compounds. Apparently, non-protein nitrogen compounds such as urea are first converted to ammonia through the action of enzymes produced by the rumen microorganisms, which then utilize the ammonia to synthesize protein amino acids which can be digested by the ruminant""s digestive system.
The ability of ruminants to utilize NPN compounds as indirect protein sources has important economic consequences. One pound of urea is equivalent in nitrogen content to over five pounds of a high protein feed such as soybean meal. On the basis of relative cost per unit of nitrogen, soybean meal is an order of magnitude more costly than urea. As a consequence, there is a strong incentive to replace a portion, or even all, of the vegetable protein fed to ruminants with NPN compounds.
Unfortunately, the amount of NPN compound that a ruminant can ingest is quite limited. The ammonia produced during post-feeding fermentation, when the NPN compound is decomposed, can be transferred to the blood stream across the rumen wall. When high levels of blood ammonia occur, the acid base balance of the blood changes and the central nervous system is affected. Early toxic systems are bloat, incoordination, labored breathing and excessive salivation; in extreme cases, there may be convulsions and death.
Various attempts have been made to limit the ammonia concentration in the rumen so as to permit the use of more NPN compound in the daily ration. Among these are several proposals for compounding or mixing urea with some kinds of water insoluble polysaccarides and/or reacting the urea in the manners above described to provide for controlled release of NPN into the rumen at rates that can be tolerated by the animal.
U.S. Pat. Nos. 3,873,733 and 4,393,082 are representative of prior developments in controlled release ruminant feed supplements, and U.S. Pat. Nos. 4,711,659 and 4,804,403 are representative of prior developments in controlled release plant nutrient formulations.
While some degree of commercial success has been achieved in respect of both plant nutrients and ruminant feed supplements, development of urea-based nutrient products that are insoluble in water, saliva and gastric juices and that have better performing time controlled rates of release remain a high priority.
U.S. Pat. No. 5,803,946, issued Sep. 8, 1998 to the assignee of the instant application, discloses controlled release plant nutrients comprising urea based particulates the surfaces of which have been transformed into a water resistant, biodegradable interpenetrating polymer network. The interpenetrating polymer network is comprised of a biuret, a urethane and tung oil interlinked one with another. The biuret is a reaction product of urea on the surface of the nutrient particles and a polyisocyanate, and the urethane is a reaction product of the polyisocyanate and an alcohol. The tung oil is cross-linked into the network by reaction with the urethane, preferably in the presence of a cross-linking initiator or promoter.
The object of the invention is to provide more reliable and better performing urea-based products having a controlled rate of release over a preselected period of time. The application is generally applicable to particulates comprised in whole or in principal part of urea and/or to particulates having urea at or on the exterior surfaces thereof, irrespective of use. However, the invention is concerned primarily with more reliable and better performing controlled release plant nutrients and controlled release NPN feed supplements for ruminant animals.
More specific objects of the invention are to provide (a) a urea based plant nutrient having a substantially linear rate of nutrient release in soil over a prolonged period of time, e.g., 30 to 120 days, and (b) a urea based ruminant feed supplement having a linear rate of NPN release in the rumen over a twelve to twenty-four hour period of time.
In accordance with the invention, improved performance is achieved by transforming the surfaces of urea based particulates into an interpenetrating polymer network coating comprised of the reaction products of urea and a polyisocyanate and the simultaneous condensation polymerization and free radical polymerization of the isocyanate, an alkyd resin having a double bond in each repeat unit and an oil, e.g., dehydrated castor oil or tung oil, having double bonds reactive with the alkyd resin double bonds and with themselves.
A three-dimensional interpenetrating polymer network shell is thus formed on the particulates at a very high level of three-dimension crosslinking density. A hydrophobic exterior protective layer is provided by alkyl chains, and offers additional water barrier properties. An optional wax overcoat may be employed to enhance handling strength.
In the resultant coating, all of the constituents are chemically bonded together and chemically bonded to the surfaces of the particles, thereby providing more reliable and better performing products.
The surfaces of the resultant particles are hard, tough, resistant to shock and abrasion, uniform, dustless and nontacky. The particles form free-flowing, essentially dustless and convenient to use urea-based products.
In the end products, the ratio of nutrients to control agents is extremely high, the controlled release is achieved at minimal expense, and the products are economical to use. In addition, the method of producing the products is economical, practical and facile.
The method may be carried out either separately from or as the final step in the urea manufacturing process, either as a continuous in-line process or a batch process. In accordance with the invention, the urea particulates are fed into a suitable apparatus, e.g., a rotary mixer, a coating drum, a fluidized bed, or the like, for creating a mobile mass of the particles within which the particles are moved in a free-falling curtain or a tumbling or rolling motion so that each particle is from time to time exposed at the surface of the mass for application thereto of a liquid or fluent polyisocyanate and a liquid or fluent mixture of alkyd resin, oil, a free radical initiator and a catalyst and/or promotor to cause the surfaces of the particles to be transformed into the interpenetrating polymer network coating above described. The method is very efficient and inexpensive to practice and produces an economical controlled release urea based product having the characteristics and features desired by industry.
These and other objects, features and advantages of the invention will become apparent to those of reasonable skill in the art from the following detailed description.